Orthogonal Frequency Division Multiplexing (OFDM) communication systems exhibit high resilience to linear distortion impairments because the information is encoded in the frequency domain. For this reason, the implementation of an optical OFDM system has become an option actively investigated, as shown in the above referenced pending U.S. patent application Ser. No. 10/679,824. However optical communication systems suffer more severely from certain impairments than radio communication systems, for which the OFDM technique was initially developed. The successful use of OFDM in the optical arena will depend on how satisfactorily it is possible to ameliorate the impact of these impairments.
One of the main limitations of optical communication systems is the optical noise generated in amplification stages which are usually needed every 50-100 km. Consequently, it is of crucial importance to establish the optimum demodulation technique in order to minimise the impact of noise without adding any unnecessary overheads.
Another limitation of coherent-type optical communication systems is the reduced coherence of the laser optical sources in comparison with their radiofrequency counterparts. It is also necessary to establish adequate phase-noise reduction techniques to be able to operate optical OFDM systems with realistic and currently available optical sources.
Orthogonal Frequency Division Multiplexing (OFDM) systems encode the information as a concatenation of blocks in the frequency domain. An inverse fast Fourier transform converts the information to the time domain before its transmission along the channel. A fast Fourier transform at the receiver recovers the original transmitted sequence.
The main advantage of OFDM is that if a guard-band with cyclic prefix is included before transmission, then the received sequence is the circular convolution of the input signal with the channel response. This permits a very easy equalisation of linear impairments, as circular convolutions become products in the Fourier domain where the information is encoded. The only limitation is that the time interval over which the channel impulse-response-time extends should be smaller than the size of the allocated guard-band.
It is also known to apply MLSE (Maximum Likelihood Sequence Estimation) to multi-carrier radio receivers using joint demodulation techniques to reduce co-channel interference (ICI). In this joint demodulation technique, symbols of the desired signal are decoded by an MLSE process at the same time as symbols of a dominant interference source. The estimated contribution of the interference source is subtracted out to decode the desired signal and the estimated contribution of the desired signal is subtracted out to decode the interference source. All possibilities of the received contribution from the desired signal and the interference source are tried, and a “score” (e.g., a viterbi decoding metric) is kept for each.
However, ICI in radio transmissions is fundamentally different to ICI in optical fibers and so radio techniques are not generally appropriate for optical systems. One such difference is that the transmission medium is dispersive in optical systems, meaning that signals of different frequencies travel at different velocities. Another fundamental difference is that the nonlinear cross coupling between intensity and phase occurs along the fiber, whereas in radio systems, there is no such distributed nonlinearity. In contrast, nonlinearities in radio systems are highly localized, being caused by a very few discrete nonlinear elements. Each such localized nonlinearity is readily compensated using a discrete compensating nonlinearity. This method is not feasible in an optical fiber system, as the fiber nonlinearity interacts with the dispersion in a complex distributed manner. Hence most cross coupling or ICI in radio systems can be modeled and pre or post compensated, whereas this is not effective for optical ICI. Also, notably the bit rates in radio systems are lower and so there is more time within the bit period, for complex processing techniques. Hence where the reach performance is limited by optical nonlinearity in the fiber, current practice is to minimise nonlinear interaction between channels by spacing the channels in frequency and using chromatic dispersion to minimise the nonlinear interaction/cross coupling (by walk-off/dephasing). Where the modulation format uses multiple-phases (such as QPSK), it is currently necessary to reduce the operating power levels to minimize the impairments due to nonlinear coupling between these nominally orthogonal phase channels.
If OFDM is implemented in optical communication systems as shown in the above referenced pending U.S. patent application Ser. No. 10/679,824, the transmitted information can be encoded in frequency by means of a given phase/amplitude modulation format like QPSK, QAM 16, QAM 32 and similar formats. The polarisation dimension can also be exploited by polarisation multiplexing. The receiver employs a coherent-type detection scheme whereby the in-phase and quadrature components of the signal are measured. Schemes that detect either only one polarisation or two orthogonal polarisations can be implemented, the second one being preferred to enhance transmission capacity. Linear impairments, like chromatic and polarisation mode dispersion, can be easily equalised once the received sampled signal is converted back to the frequency domain to extract the original data.
Standard OFDM for radio systems offers two approaches for data demodulation, either in a pilot-tone based scheme or in a differentially-detection scheme:                Pilot-tone scheme: reference pilot-tones with known data are transmitted along with the desired information, and used in the receiver to recover a reference signal for each frequency channel (subcarrier) to demodulate the data. The reference signal is built along time by an averaging process, so that it is virtually noiseless, providing good resilience against additive white Gaussian noise as it attains the coherent detection limit. However, the transmission of pilot tones containing known information constitutes an overhead that wastes part of the channel capacity, and which is not necessary in the differentially-detection scheme. Also the complexity of the receiver will be high.        Differential detection scheme: In this scheme the data is demodulated by comparing consecutive sampled symbols corresponding to the same frequency channel. There is a performance difference between the two schemes in relation to their resilience to additive white Gaussian noise (AWGN), and the possibility of phase error correction. No overhead is required but it exhibits a signal-to-noise ratio penalty with respect to pilot tones. No phase error correction technique has been implemented in this scheme.        
It is known from EBU technical review summer 1998, J Stott, “The effects of Phase noise in COFDM”, that there are two types of phase noise, a common phase error common to all the channels and a thermal noise-like part which is not pure phase noise. COFDM is coded orthogonal frequency division multiplexing. It shows that a pilot tone scheme can be implemented with phase error correction circuitry in order to correct for common phase errors in a digital video broadcast system.
In optical systems, AWGN introduced at each amplification stage is usually one of the performance limiting factors. The OFDM-pilot-tone based scheme exhibits the same Bit Error Rate (BER) versus Optical Signal-to-Noise Ratio (OSNR) performance as its equivalent coherently detected modulation formats. Analogously, the OFDM-differentially-detected scheme also exhibits the same BER-OSNR performance as its equivalent differentially-detected modulation format. For instance, coherent-QPSK detection has an OSNR margin of ˜2.4 dB at a BER of 10−3 with respect to differential-QPSK.